Hack with confidence.
Page document-viewer.php is vulnerable to at least the following:
HTML Injection: It is possible to inject your own HTML into this page because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject HTML, CSS, and/or Javascripts in order to alter the client-side code of this page.
HTTP Parameter Pollution: If multiple parameters with the same name are sent in a request, different application servers will react differently. PHP takes only one of the parameters but not neccesarily the parameters intended by the developer. By duplicating parameters with a value of your choosing and placing that parameters before and-or after the pages native parameters, you can influence the pages behavior. Note that ASP and Java application servers act different.
Method Tampering: Because the page does not specify that the input parameters must be posted, it is possible to submit input parameters via a post or a get. This is a second order vulnerability allowing other vulnerabilities to be exploited easier.
Frame Source Injection: By controlling the parameter which determines the src attribute of a pages frame, a carefully injected value can load any arbitrary page into the frame.
Application Log Injection: Some inputs on this page are recorded into log records which can be read by visiting the Show Log page. Vulnerabilities on the Show Log page may allow injections in log records to execute.
Reflected Cross-Site Scripting: This page is vulnerable to reflected cross-site scripting because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject scripts. Try to redirect the user to the capture-data.php page which records cookies and other parameters. Visit the captured-data.php page to view captured data.